Method in the manufacture of stainless, hardenable chromium-steel strip and sheet

ABSTRACT

There is provided a method of producing stainless, chromiumsteel sheet or strip having a high resistance to corrosion and an improved hardness, and which has a relatively high yield point / ultimate strength ratio in an unhardened state, the starting material used in the method being one of substantially a pearlite structure and in which the material is heated and worked at specific temperatures so as to break down the pearlite structure and transform said structure to one comprising finely-divided spheroidized carbides in a ferritic matrix.

United States Patent Jakenberg 51 May 2,1972

[54] METHOD IN THE MANUFACTURE OF STAINLESS, HARDENABLE CHROMIUM-STEEL STRIP AND SHEET [72] Inventor: Klas-Erik Jakenberg, Hagfors, Sweden [73] Assignee: Uddeholms Aktiebolag, l-lagfors, Sweden [22] Filed: May 23, 1969 [21] Appl. No.: 827,355

[30] Foreign Application Priority Data May 31, 1968 Sweden ..7333/68 [52] U.S.Cl ..148/12, 148/12.3, 148/37 [51] Int. Cl. ..C21d 9/18, C22c 39/14 [58] Field of Search ..148/12, 37, 12.3, 124; 75/126 [56] References Cited UNITED STATES PATENTS 2.801.916 8/1957 Harris et al. ..148/12 Harris et a1. ..148/12 Ma1zacher..... ..148/12.4

3,216,868 11/1965 Nachtman 148/12 3,281,287 10/1966 Edstrom 148/1 2.4 3,425,877 2/1969 Deacon ..148/124 Primary Examiner-J... Dewayne Rutledge Assistant Examiner-W. W. Stallard Att0rneyPierce, Scheffler & Parker [5 7] ABSTRACT 6 Claims, 8 Drawing Figures METHOD IN THE MANUFACTURE OF STAINLESS, HARDENABLE CHROMIUM-STEEL STRIP AND SHEET Cold rolling has hitherto been used in the production of thin, stainless strip and sheet metal; i.e. a conventional technique. The starting material has comprised a hot-rolled material, surface treated by, for example, pickling, grinding, blasting or the like operations. The material has then been cold-rolled in a large number of passes, alternating with anneals and intermediate anneals, until the desired dimensions are obtained. Although this method is a lengthy one, and thus expensive, it is still applied in practice, since it is considered the only conceivable method by which a high-quality product can be obtained. Cold rolling namely increases the properties of the steel in several respects; for instance its hardness and mechanical strength. The surface of the metal becomes bright and thus, among other things, more resistant to corrosion.

The cold-rolling technic, however, is accompanied with marked disadvantages. One such disadvantage is the longdrawn process required by the technic; which plays a dominating role in the final price of the product. When concerning stainless steels, there are also difficult problems to resolve in connection with the annealing operations rendered necessary by the cold hardening of the steel. Those factors which influence the quality of the final product, are, in addition to the nature of the starting material, primarily the number of anneals necessary, the manner in which the anneals are effected and the annealing temperatures and times.

There is little that can be done about the number of anneals since this depends upon the requisite reduction in area. With regard to the manner in which the anneals are effected, it is important that the strip or sheet is heated uniformly over the entire surface thereof and through the whole of its cross-section; something which cannot be fully accomplished on production scales. Repeated heating of stainless steel to temperatures within the range of 700800 C and subsequent coolingcauses considerable variation in the structure of the steel. The resulting inhomogeneities in the metal, possibly combined with irregularities resulting from theheat treatment thereof, have been given as the reason why the hardness of cold-rolled, stainless steel varies considerably after hardening. This is particularly the case when hardening is effected continuously and only limited time is at disposal for the solution heat treatment of the steel. The desideration is a product which is uniform in all respects; a desire which in many cases is impossible to realize with the technic used hitherto.

Another disadvantage caused by the inhomogeneities arisingas a result of heat treatment, is the uneveness of the material, e.g. so-called dishing, owing to the state of the internal stresses. A further, serious disadvantage associated with repeated annealing processes is that such processes result in an undesirable coarseness of the carbide grains. Attempts have been made to restrain this negative effect, by maintaining the lowest possible temperature, whereby the growth rate of the carbides can be reduced. Nevertheless, carbide grains may be formed which are so large and of such a nature that in conjunction with a continuous hardening sequence they are not able .to dissolve completely during the austenitizing process. The result is that in some cases the degree of hardness possible in theory cannot be reached, adding yet another factor to the variation in hardness. The fact that complete dissolution is not always obtained can also affect the resistance of the steel to corrosion, since part of the chromium becomes bound in the form of unsolubilized chromium carbides. The non-rust properties of the steel can naturally be improved by increasing the amount of chromium in the analysis, or by increasing the time taken to austenitize the steel. The first alternative, however, is deleterious to the price of the final product whereas the latter alternative affords a reduction in capacity, which for natural reasons is not to be desired.

It can be said of the present standpoint of technics with regard to stainless, cold-rolled steel, that materials available today present qualitative weaknesses while simultaneously demanding a high price as a result of complicated manufacturing processes.

The first object of the invention is to produce a stainless, strip or sheet steel which is highly resistant to corrosion and having improved hardness in relation to known steels of corresponding composition. By improved hardening is meant an increased ability of the carbon to rapidly solubilize during austenitizing of the steel.

Another object of the invention is to produce sheet or strip steel of homogeneous structure.

A further object of the invention is to produce stainless, strip or sheet having a relatively high degree of flatness.

One object of the invention is also to produce a material having a high yield point ultimate strength ratio in an unhardened state.

Still another object of the invention is to produce a steel with fewer variations in hardness after hardening than has previously been possible.

These and other objects are realized by means of the invention, which is mainly characterized by using as a starting material a material whose main structure is pearlite, by heating the material to a temperature of between 600 and 790 C and rolling the material in this temperature range, by adapting the working temperature and degree of reduction so that the pearlite structure is broken down and transformed to a structure substantially comprising finely-divided spheroidized carbidesin a ferritic matrix having at least 40, preferably at least 60, and in certain cases more than carbide grains per l00,u,m

By hardenable, stainless chromium-steel is meant such hardenable, stainless steels which in addition to iron contain chromium as the dominating alloying constituent. The carbon content of these steels varies from between about 0.1 percent and about 1.0 percent. Remaining alloying constituents are present in moderate percentages (each at a maximum of about 2 percent) and may, for example, comprise silicon, manganese, molybdenum, nickel and copper. Other elements, such as boron, beryllium, nitrogen etc, may also be present in small quantities.

It is known to roll strip at the temperatures in question. For instance, it was suggested almost fifty years ago to roll carbonsteel strip at a temperature of 600 C, to appropriate the ductility at this temperature. The method, however, has not been used in practice, either withcarbonsteel or with stainless steel, which can be attributed to the fact that the significance of a homogeneous starting structure prior to rolling was not recognized. A homogeneous starting structure has proved of particular importance in the case of stainless steel, whose structure is, after all, more complicated and can therefore be deranged more easily than the structure of normal carbon steel. The structure should preferably comprise substantially of pearlite, whereby is meant the austenite decomposition or transformation product lamellar or not which is formed in the temperature range of 790-580 C. In addition, the structure should be homogeneous without appreciable admixture, by which is meant maximum of 20 percent of other structure constituents than the main structure.

In the method of the invention, the starting material is hotrolled strip which has been cooled to room temperature and surface treated by, for example, pickling, grinding, blasting or like processes. The strip is preferably cooled while well coated in insulating ashes, to ensure the pearlitic structure. The steel can normally be worked direct, without prior annealing being necessary, and to unlimited reduction, by which is meant that the reduction is so extensive that the desired final dimension can be obtained in the absence of intermediate anneals. By area reduction is understood the ratio between the reduction in area of the strip or sheet at right angles to the direction of stretch and the corresponding starting area. In spite of the comparitively high working temperature bright surfaces are obtained and scale formation is negligible. A protective atmosphere which is conceivable in principle has not proven necessary. Subsequent to being rolled in accordance with the invention, the material can also be rolled at room temperature, for the purpose of achieving certain tolerances for instance.

The surprising results obtained by treating stainless, hardenable steel according to the method of the invention will now be described with reference to the following example, reference also being made to the accompanying photographs, which illustrate the structure of the steel at different working stages.

EXAMPLE The test was concerned with the production of stainless steel strip having properties which in several respects were superior to those of material produced by cold rolling according to conventional methods. The analysis of the steel was 0.66 percent C, 0.38 percent Si, 0.45 percent Mn, 13.8 percent Cr, 0.06 percent Ni, 0.04 percent Mo and 0.063 percent N. The material had been first hot rolled in a conventional manner, down to a thickness of 6 mm, slowly cooled, buried in ashes, to room temperature and then pickled.

Subsequent to the pickling operation, the material was heated in anelectric furnace provided with a movable hard bottom positioned as close as possible to the roll gap. The temperature of the furnace was varied from between, 600 and 750 C. The basic structure of the steel is still ferritic at these temperatures. Immediately upon obtaining the desired temperature, the material was rolled direct to the final dimension without intermediate anneals. With regard to ductility, it was discovered that there was a temperature limit range between 600 and 650 C. For the type of steel in question the lower limit can be taken as being about 620 C. Ductility was good from 650 C up to and including 750 C, and strong reductions were obtained at each pass, and hence only a few passes were necessary. In the laboratory roll mill at disposal reductions of up to 44 percent could be made in one single pass, a magnitude, however, which should not be considered a maximum for the technic in question. After each pass, a sample, length 2 dm, was cut from the material. Microtests were then made on the samples for the purpose of examining the structure. Hardness and corrosion tests were made on rolled and hardened material.

FIGS. 1, 2 and 3 1,200 X) show the starting structure of the steel and the structures after a 67 percent and 76 total area reduction at a rolling temperature of 750 C. Rolling should be continued to an extentwhereby finely-divided spheroidized carbides are obtained with at least 40 and preferably at least 60 carbide grains per 100 um In addition the structure should be homogenous, without appreciable rests of pearlite. The carbide content was satisfactory after a 67 percent reduction in area (FIG. 2), although small portions containing pearlitic residues were still present. Subsequent to rolling down to a 76 percent reduction (FIG. 3) no pearlitic portions could be observed. The carbidecontent was very high; well in excess of the highest value, 80 carbides per 100 [1.111 of the carbide scale.

To illustrate the significance of a highly homogeneous, pearlitic starting material, there is shown in FIG. 4 1,200 X) a starting material of the same analysis as the previous material but with a mixed structure, while FIG. 5 (1,200 X) shows the same material rolled to an approximate 78 percent total reduction at 700 C. The starting structure, FIG. 4, is comprised of a mixed structure of complex character, and the final structure, FIG. 5, hereby obtains an uneven carbide distribution. The Figure clearly shows sweeps of material having fewer carbides.

FIG. 6 (1,200 X) illustrates a starting material which is partly spheroidized and has an uneven structure. FIG. 7 shows the same material rolled to a total area reduction of 77 percent at 750 C. Undesired sweeps of portions containing fewer carbides are also present in this instance. The spheroidizing, however, is sufiicient.

Subsequent to rolling at the aforementioned high temperature the strip is preferably further reduced in thickness by a conventional cold rolling process. This operation is normally necessary to give desired tolerances, although it also improves the quality of the steel insofar as the yield point ultimate strength ratio rises to a value which is difficult to achieve when using solely conventional technics. This operation is not carried out in this example, however, but instead the strip is hardened directly after rolling at said high temperature.

Hardening of the strip is effected in a simulator, adapted to simulate operational conditions in continuous hardening processes. In such hardening processes the strip is subjected to solubilizing treatment continuously upon passage through a furnace heated to a temperature of l,l00 C. The time at disposal with the existing strip speeds is 45 seconds. The strip is then cooled continuously at 70 C.

The hardness was tested for material according to the invention which had been rolled at 650, 700 and 750 C and which was then hardened at varying stay times during the austenitizing treatment process. The reference material comprised two samples of the same composition as the material of the invention but produced by conventional cold rolling technics. The reference material was also hardened under the same conditions as the material produced according to the invention. It was discovered that as opposed to the reference material the materials produced according to the invention showed very little spread while at the same time they lay on a higher level. These circumstances can be attributed to the fine, readily solubilizable and homogeneous structure of the material according to the invention, but are nevertheless remarkable. The following values were obtained.

TABLE I Hardness in Vickers units (20X5 kp); austenitizing temperature l,l00 C; cooling to -70 C.

Stay times (secs) at 1,100 0. Rolling te In FIG. 8 the hardness has been written as the function of the stay time for the two reference steels and the two steels rolled at 700 C, i.e. the materials nos. 612 and 71 1. In order to illustrate the spread graphically, the space between associated curves has been shaded. The Figure shows with all cleamess the superiority of the material of the invention with regard to uniformity in hardness, and that the material of the invention reaches its maximum level more rapidly and that this level is higher than that of the reference material.

In addition to an improved hardness, good results were also obtained in other respects. For instance, the degree of flatness of the material was good; probably owing to its homogeneous structure. The resistance of the material to corrosion was also good, and the punchability of the material was very good sub sequent to passing said material through finishing rolls, owing to a high yield point/ ultimate strength ratio.

Although the invention has been, described with reference to one example thereof, it should be understood that it is not restricted thereto and can be varied within the scope of the claims. The steel compositions are only to be considered conceivable examples of types of analyses which can be used in accordance with the invention. Thus, the steel should have a carbon content in excess of 0.1 percent and preferably in excess of 0.2 percent. In addition to carbon and chromium other additives usually used in stainless steel may be employed. Material which contains such elements as those which favor the occurrence of graphite, e.g. silicon, nickel or aluminium,

i.e. material which in conventional technics gives rise to great difficulties in connection with cold working and annealing processes, can be used to advantage in accordance with the invention. It is also possible to manufacture steel which contains less slag than steel which tends to form graphite upon being treated conventionally, by adding the deoxidation agents aluminium and silicon in quantities of up to about 0.1 percent and 2 percent respectively.

What is claimed is:

l. A method for the manufacture of hardenable steel strip and sheet which comprises, hot rolling a billet of the steel in the austenite condition, slowly cooling the hot-rolled strip or sheet to room temperature to produce a structure in the rolled material containing at least 80 percent pearlite, heating the material to a temperature between 600 and 790 C, and rolling in the ferritic state of the steel matrix to a total area reduction of at least 70 percent, so that the pearlite is broken down and transformed to a structure of finely-divided spheroidized carbides in a ferritic matrix having at least 40 carbide grains per 100 m 2. A method according to claim 1, wherein the hot-rolled material is cooled to room temperature in an insulating medi- 3. A method according to claim 1, in which the steel is a stainless, hardenable chromium steel in which rolling of the material with the pearlitic structure is effected in the temperature range from 700 to 780 C.

4. A method according to claim 1, characterized in that the steel is rolled at a temperature of about 700 C.

5. As a new product, hardenable steel strip or sheet containing finely divided spheroidized carbide particles in a ferritic matrix having in excess of 40 such particles per pm*, said steel strip or sheet having been manufactured by the method defined in claim 1.

6. A steel strip or sheet according to claim 5, characterized in that the steel matrix has an alloying composition essentially consisting of from 0.2 to 1.5 weight-percent carbon, 5 to 20 weight-percent chromium, O to 4 weight-percent of an alloying metal selected from the group consisting of Ni, Mo, ,Mn, Cu and Co, 0 to 3 weight-percent of an alloying element of the group consisting of W and Si, 0 to 2 weight-percent of V, and 0 to 1 weight-percent of Ti, Ta, Nb, B and Be. 

2. A method according to claim 1, wherein the hot-rolled material is cooled to room temperature in an insulating medium.
 3. A method according to claim 1, in which the steel is a stainless, hardenable chromium steel in which rolling of the material with the pearlitic structure is effected in the temperature range from 700* to 780* C.
 4. A method according to claim 1, characterized in that the steel is rolled at a temperature of about 700* C.
 5. As a new product, hardenable steel strip or sheet containing finely divided spheroidized carbide particles in a ferritic matrix having in excess of 40 such particles per 100 Mu m2, said steel strip or sheet having been manufactured by the method defined in claim
 1. 6. A steel strip or sheet according to claim 5, characterized in that the steel matrix has an alloying composition essentially consisting of from 0.2 to 1.5 weight-percent carbon, 5 to 20 weight-percent chromium, 0 to 4 weight-percent of an alloying metal selected from the group consisting of Ni, Mo, Mn, Cu and Co, 0 to 3 weight-percent of an alloying element of the group consisting of W and Si, 0 to 2 weight-percent of V, and 0 to 1 weight-percent of Ti, Ta, Nb, B and Be. 